1. Field of the Invention
This invention is concerned with upgrading relatively heavy hydrocarbon streams by converting a portion or all of the charge to lower molecular weight materials. Embodiments of this invention include cracking or hydrocracking of heavy petroleum fractions to produce gasoline and distillate, and catalytic dewaxing wherein the wax portion of the feed is selectively converted to lower molecular weight materials. All of these conversions involve "cracking" in the broadest sense of this term since all of these conversions depend on the conversion of hydrocarbon or substituted hydrocarbon molecules to products of lower molecular weight.
2. Background and Prior Art
Cracking may be nonspecific in that various types of molecules are converted, i.e., branched and straight chain aliphatics, naphthenes, aromatics, etc. The compounds so converted may also include other atoms in the molecule: metals, oxygen, sulfur and/or nitrogen. In particular processes, however, the intent may be to convert a certain class of compounds in order to modify a characteristic of the whole. Exemplary of the latter type of conversion is shape selective conversion of straight and slightly branched aliphatic compounds of 12 or more carbon atoms to reduce the pour point, the pumpability and/or the viscosity of heavy fractions which contain these waxy constituents. The long carbon chain compounds tend to crystallize on cooling the oil, and in many cases such cooled oil will not flow, hence may not be pumped or transported by pipelines. The temperature at which such mixture will not flow, which is determined by standarized test procedures, is designated the "pour point".
The pour point problem can be overcome by techniques known in the art for removal of waxes or conversion of those compounds to other hydrocarbons which do not crystallize at ambient temperatures. Shape-selective cracking or hydrocracking is an important method for so converting waxy hydrocarbons, utilizing principles described in U.S. Pat. No. 3,140,322 dated July 7, 1964. Zeolitic catalysts described in the literature for selective conversions of wax include such species as mordenite, with or without added metal to function as a hydrogenation catalyst.
Particularly effective catalysts for catalytic dewaxing include zeolite ZSM-5 and related porous crystalline aluminosilicates as described in U.S. Pat. No. Re. 28,398 (Chen et al.) dated Apr. 22, 1975. As described in that patent, drastic reductions in pour point are achieved by catalytic shape selective conversion of the wax content of heavy stocks with hydrogen in the presence of a dual-functional catalyst of a metal plus the hydrogen form of ZSM-5. The conversion of waxes is by scission of carbon to carbon bonds (cracking) and production of products of lower boiling point than the waxes. However, in many instances only a small portion of the charge is converted in dewaxing. For example, Chen et al. describe hydrowaxing of a full range shale oil having a pour point of +80.degree. F. to yield a pumpable product of pour point at -15.degree. F. The conversion of materials from the fraction heavier than light fuel oil to lighter components was in the neighborhood of 9%.
Among the less selective techniques for producing products of lower molecular weight than the hydrocarbon charge stock are catalytic cracking and catalytic hydrocracking. Catalytic cracking involves contacting the heavy hydrocarbon charge with a porous acidic solid catalyst at elevated temperatures in the range of 850.degree. to 1000.degree. F. to yield the desired lower boiling liquid product of greater value than the liquid charge (e.g. motor gasoline) together with normally gaseous hydrocarbons and coke as by-products. Hydrocracking employs a porous acidic catalyst similar to that used in the catalytic cracking but associated with a hydrogenation component such as metals of Groups VI and VIII of the Periodic Table. An excess of hydrogen is supplied to the hydrocracking reactor under superatmospheric pressure at lower temperature than those characteristic of catalytic cracking, say about 650.degree. F.
Since the introduction of zeolite catalysts as exemplified by U.S. Pat. No. 3,140,249, a large proportion of the capacity for catalytic cracking and hydrocracking has been converted to use of such highly active catalysts. The high activity zeolite catalysts are characterized by very low content of alkali metal. Sodium, for example, is present as a cation in synthetic faujasites by reason of their manufacture. Expensive ion exchange operations are carried out in the preparation of cracking and hydrocracking catalysts from synthetic faujasite to replace the sodium or other alkali metal by protons or polyvalent metal cations.
It has been recognized that such zeolites can function as catalysts when containing a moderate percentage of sodium. Thus Kimberlin and Gladrow U.S. Pat. No. Re. 26,188 exhibits data showing cracking activity of a faujasite from which only one-third of the sodium has been removed by ion exchange. The extremely high activity of such catalysts as zeolite ZSM-5 has been moderated for specialized purposes by using the zeolite in the partially sodium form. See, for example, U.S. Pat. No. 3,899,544.
Zeolite ZSM-5 preparation is described in U.S. Pat. No. 3,702,886 which also describes several processes in which the zeolite is an effective catalyst, including cracking and hydrocracking. That zeolite is shown to be prepared from a forming solution which contains organic cations, namely alkyl substituted ammonium cations, which occupy cationic sites of the zeolite after crystallization. It is conventional to remove the organic cations by burning in air at elevated temperature, after which ion-exchange with sodium or other cations may be effected.
In general, the principal products or by-products desired in cracking, hydrocracking and related processes are relatively low-boiling liquids such as motor gasoline, diesel fuel, jet fuel, No. 2 fuel oil and the like, which liquids collectively will be referred to herein as gasoline plus distillate, or simply G+D. Gaseous products such as hydrogen, methane, ethane, propane, etc. represent degradation of a portion of the charge to less valuable fuels that the desired premium products. In addition to being less valuable fuels, these gases sequester high proportions of hydrogen, thus depriving other fuel fractions of what is ordinarily a desirable component.
Ways have been sought to increase the G+D (gasoline plus distillate) selectivity in cracking and hydrocracking, with reduction of the amount of feed going to gases. In U.S. Pat. No. 4,263,129 issued Apr. 21, 1981, it is proposed to conduct hydrocracking, hydrodewaxing, and related processes with certain zeolites having a low acid activity as catalyst. The low acid activity is achieved by alkali metal exchange, although it may also be achieved by steaming or using zeolites of very high silica to alumina ratio. In particular, low acidity catalysts that have an acid activity measured by the alpha scale less than 10 are required, with an alpha value substantially lower than unity being preferred. In U.S. Pat. No. 4,284,529 it is proposed to conduct similar conversions but with low acidity catalysts prepared by alkali metal exchange followed by steaming. Both U.S. Pat. No. 4,263,129 and U.S. Pat. No. 4,284,529 use as catalyst crystalline zeolites having a silica to alumina ratio greater than 12. When used as described, the low acid activity catalysts produce reduced amounts of gaseous by-products compared with prior art catalyst, and, additionally require less frequent regeneration. The entire contents of U.S. Pat. No. 4,263,129 and of U.S. Pat. No. 4,284,529 are incorporated herein by reference.
The acid activity of zeolite catalysts is conveniently measured by the alpha scale described in an article published in Journal of Catalysis, Vol. VI, pp. 278-287 (1966), the entire content of which article is herein incorporated by reference. The alpha scale so described will be used herein to define activity levels.
Crystalline zeolites, as synthesized, are fine powders unsuited for direct use as catalyst in almost all instances. The powders may be compacted under high pressure to form tablets of acceptable particle size. As a practical matter, however, the manufacture of acceptable catalysts from zeolite powders usually is accomplished with the use of a matrix, i.e. a second substance which is chosen for ease of handling, cost, ability to impart good crush strength to the catalyst, as well as for other factors. Known matrix materials include clays, alumina, amorphous cogels of silica with alumina (silica-alumina), silica-titania, silica-magnesia, and silica-zirconia. Means for combining the zeolite with the matrix material and for forming catalyst of the correct particle size from the mixture are known to those skilled in the art of catalyst manufacture and need not be reviewed here.
It is an object of this present invention to provide a selective process for upgrading a hydrocarbon oil with reduced formation of gaseous by-product. It is a further object of this invention to provide a process wherein a high boiling hydrocarbon oil to be upgraded is contacted with a low acidity catalyst comprising a heat stable zeolite in a matrix of collapsed heat-sensitive zeolite. These and other objects will become apparent to one skilled in the art on reading this entire specification and the claims appended hereto.